Spatial division multiple access (SDMA) that multiplexes a plurality of users at the same frequency and the same time by using a plurality of transmit antennas is known as a technology that can effectively exploit frequency resources. There is known zero-forcing (ZF)-SDMA that applies this technology to a wireless communication system which performs communication with a base station and a plurality of users (wireless terminals) and enables reception of transmitted signals for the respective users alone without interfering with the plurality of users.
ZF-SDMA is a criterion that suppresses interferences from other users, and it multiplies a transmitted signal by a weight W corresponding to an inverse matrix of a matrix representing a state of a plurality of propagation paths between a plurality of transmit antennas and a plurality of users (which will be referred to as a channel matrix hereinafter) to effect beam forming in a base station, thereby suppressing interferences. Since ZF-SDMA uses a degree of freedom of each transmit antenna for suppression of interferences in this manner, transmission power is increased and excellent diversity performance cannot be obtained. That is, considering a user 1 and a user 2 as wireless terminals, when it is assumed that a zero direction of a transmission beam is set to, e.g., a direction of the user 2 in order to suppress an interference with the user 2 with respect to a transmitted signal for the user 1, a maximum amplitude direction of a transmission beam for the user 2 does not necessarily coincide with a direction of the user 2.
In G. Caire and S. Shamai (Shitz), “On the achievable throughput of a multiantenna Gaussian broadcast channel,” IEEE Trans. on Info. Theory, vol. 49, No. 7, pp. 1691-1706, July 2003. (Document 1), DPC (Dirty Paper Coding)-SDMA that solves this problem is proposed. In the DPC-SDMA scheme, Q obtained by performing QR decomposition with respect to HH that is a Hermitian transposition of a channel matrix H is used as a weight W for beam forming. Since Q is an orthogonal matrix, an increase in transmission power that is a problem in ZF-SDMA does not occur. When a transmitted signal is multiplied by the weight W based on beam forming, the wireless terminal of the user 1 receives a transmitted signal for the user 1 alone, and the wireless terminal of the user 2 receives both the transmitted signal for the user 1 and a transmitted signal for the user 2.
In this case, a signal is transmitted to the user 1 with a transmission beam that provides a maximum gain in the wireless terminal of the user 2. Therefore, the interference does not have to be suppressed with respect to the user 1, and hence transmission with a beam synthesized at a maximum ratio is enabled, thus improving performance beyond those of ZF-SDMA. At this time, a diversity order of the wireless terminal of the user 1 becomes 2. Here, since the transmission beam for the user 1 does not take the wireless terminal of the user 2 into consideration, the transmitted signal for the user 1 is also transmitted to the user 2. On the other hand, the transmitted signal for the user 2 does not reach the wireless terminal of the user 1 since the interference with the wireless terminal of the user 1 must be suppressed, but this transmitted signal does not reach the wireless terminal of the user 2 with a maximum gain either, and a diversity order of the wireless terminal of the user 2 becomes 1.
The transmitted signal for the user 1 interferes with the wireless terminal of the user 2 if nothing is done. To avoid this interference problem, precoding based on feedback processing is performed with respect to each transmitted signal in DPC-SDMA. Using both beam forming and precoding in this manner allows the wireless terminals of the user 1 and the user 2 to communicate with the base station without interferences. In this case, the diversity order of the wireless terminal of the user 1 becomes 2, and the diversity order of the wireless terminal of the user 2 becomes 1. On the other hand, in ZF-SDMA, both the users have the diversity order of 1. Therefore, it can be said that DPC-SDMA is a scheme that is superior to ZF-SDMA. However, DPC-SDMA has a problem that transmission power is increased when precoding is effected for elimination of interferences alone.
On the other hand, in C. Windpassinger, R. Fischer, T. Vencel, and J. B. Huber, “Precoding in multiantenna and multiuser communications,” IEEE Trans. Wireless Communication, vol. 3, No. 4, pp. 1305-1316, July 2004. (Document 2), a transmission power reduction method using Tomlinson-Harashima Precoding (THP) is proposed. This is a method of performing such modulo arithmetic as provided in THP to reduce transmission power. This scheme will be referred to as THP-SDMA hereinafter. THP-SDMA can multiplex users at the same time and the same frequency without increasing transmission power and improve a reception quality as compared with general SDMA schemes.
When decoding a signal transmitted based on THP-SDMA on a reception side, the received signal must be divided by a gain (a reception gain) corresponding to a channel response on the reception side. Since the gain corresponding to the channel response is not known on the reception side, a pilot signal must be used from a transmission side to inform the reception side of the gain in advance. In this case, how the pilot signal is transmitted in THP-SDMA is a problem.
It is known that, when transmitting the pilot signal in conventional ZF-SDMA, the same weight as that used in transmission of a data signal for beam forming is utilized. When the same technique is applied to THP-SDMA, performing the modulo arithmetic with respect to the pilot signal to avoid an increase in transmission power results in occurrence of the following problem.
When performing channel estimation using a pilot signal, a received signal is divided by a known pilot signal to obtain a channel estimated value on the reception side. However, when the modulo arithmetic is carried out on the transmission side to pilot signals, the channel estimated value greatly deviates from an original value, thus extremely degrading reception performances.
On the other hand, when a pilot signal is transmitted without performing the modulo arithmetic, a channel estimated value can be correctly calculated, but increased transmission power must be normalized, and hence a reception level of the pilot signal is consequently reduced, thus resulting in a problem that a reception quality is likewise degraded.